Mayulestes ferox, a borhyaenoid (Metatheria, Mammalia) from the early Palaeocene of Bolivia. Phylogenetic and palaeobiologic implications

C Muizon, De Geodiversitas
1998 Geodiversitas   unpublished
Mayulestes ferox is a borhyaenoid marsupial from the early Palaeocene of Tiupampa (Bolivia). The holotype and only known specimen is a partial ske leton which is described and discussed below. Mayulestes ferox is a member of the family Mayulestidae, a taxon which also includes the species Allqokirus australis from the same locality and age, but which is only known by a few isolated molars. Mayulestes and Allqokirus are the two oldest known borhyaenoids. Mayulestes differs from Allqokirus in the
more » ... morphology and proportions of its molars. A major feature of the molars of both genera is the reduction of the entoconid which is regarded here as a synapomorphy of the Mayulesti dae. Mayulestes has the plesiomorphic marsupial dental formula (I5/i4; Cl/cl; P3/p3; M4/m4) and its molar morphology approaches the plesio morphic marsupial cheek tooth pattern. Mayulestes ferox does not have a tympanic process of the alisphenoid, a structure whose presence is generally regarded as a marsupial synapomorphy. Comparison with other borhyaenoid taxa indicates that the lack of tympanic process of the alisphenoid is in fact a plesiomorphic character state for the superfamily, and it is suggested that this feature appeared several times during marsupial evolution. The ear region of Mayulestes bears a conspicuous medial process of the squamosal and there is a shallow cavity (the roof of the alisphenoid sinus) between the foramen ovale and the glenoid cavity, excavated within the squamosal anteriorly, the pério de posteriorly, and the alisphenoid between. The contribution of the squa mosal to the roof of the alisphenoid sinus is regarded as the key synapomorphy of the borhyaenoids. Other borhyaenoid synapomorphies are: the loss of the prootic canal, the reduction and the loss of the antero lateral process of the maxilla, and the probable loss of epipubic bones. The postcranial skeleton of Mayulestes is represented by twenty complete or par tial vertebrae, a few ribs and most major limb bones. A comparison with living didelphids, Pucadelphys, other borhyaenoids, and several arboreal (or probably arboreal) mammals such as sciurids, tupaiids, procyonids, multituberculates morganucodontids, triconodontids, and Henkelotherium reveals that many features of the postcranial skeleton of Mayulestes are indicative of • 1998 • 20 (1) GEODIVERSITAS • 1998 • 20(1) Mayulestes, a borhyaenoid from the Palaeocene of Bolivia MOTS CLÉS Marsupialia, Borhyaenoidea, Paléocène, Bolivie, phylogénie, anatomie fonctionnelle. HYPODIGM. -Type specimen only. LOCALITY, HORIZON AND AGE. -The specimen was collected on the site 1 ("the quarry"), at the locality of Tiupampa, situated about 95 km southeast of Cochabamba (Mizque Province, Department of Cochabamba, Bolivia). Site 1 (see Gayet et al